23 research outputs found
Exotic-singularity-driven dark energy
We discuss various types of exotic (non-standard) singularities in the
Universe: a Big-Rip (BR or type I), a Sudden Future Singularity (SFS or type
II), a Generalized Sudden Future Singularity, a Finite Scale Factor singularity
(FSF or type III), a Big-Separation (BS or type IV) and a -singularity. They
are characterized by violation of all or some of the energy conditions which
results in a blow-up of all or some of the physical quantities: the scale
factor, the energy density, the pressure, and the barotropic index. We relate
the emergence of these singularities with physical theories (superstring,
brane, higher-order gravity, loop quantum cosmology). We show how the models
involving exotic singularities may serve as dark energy by applying the
observational data. In particular, we show that some of these exotic
singularities (though being of a weak type according to relativistic
definitions) may occur in the near future of the universe.Comment: 10 pages, 9 figures, contribution to "Invisible Universe", paris
29.06-3.07.09, to appear in AIP Conference Proceeding
Are we survivors of the sudden past singularity?
In this paper, we investigate the viability of cosmological models featuring
a type II singularity that occurs during the past evolution of the Universe. We
construct a scenario in which the singularity arises and then constrain the
model parameters using observational data from Type Ia Supernovae, Cosmic
Chronometers, and Gamma Ray Bursts. We find that the resulting cosmological
models based on scenarios with the past type II singularity cannot be excluded
by kinematical tests using current observations.Comment: 5 pages, 1 figur
How far is it to a sudden future singularity of pressure?
We discuss the constraints coming from current observations of type Ia
supernovae on cosmological models which allow sudden future singularities of
pressure (with the scale factor and the energy density regular). We show that
such a sudden singularity may happen in the very near future (e.g. within ten
million years) and its prediction at the present moment of cosmic evolution
cannot be distinguished, with current observational data, from the prediction
given by the standard quintessence scenario of future evolution. Fortunately,
sudden future singularities are characterized by a momentary peak of infinite
tidal forces only; there is no geodesic incompletness which means that the
evolution of the universe may eventually be continued throughout until another
``more serious'' singularity such as Big-Crunch or Big-Rip.Comment: REVTEX4, 4 pages, 2 figures, references change
A critical assessment of some inhomogeneous pressure Stephani models
We consider spherically symmetric inhomogeneous pressure Stephani universes,
the center of symmetry being our location. The main feature of these models is
that comoving observers do not follow geodesics. In particular, comoving
perfect fluids have necessarily a radially dependent pressure. We consider a
subclass of these models characterized by some inhomogeneity parameter .
We show that also the velocity of sound, like the (effective) equation of state
parameter, of comoving perfect fluids acquire away from the origin a time and
radial dependent change proportional to . In order to produce a
realistic universe accelerating at late times without dark energy component one
must take . The redshift gets a modified dependence on the scale
factor with a relative modification of peaking at and
vanishing at the big-bang and today on our past lightcone. The equation of
state parameter and the speed of sound of dustlike matter (corresponding to a
vanishing pressure at the center of symmetry ) behave in a similar way and
away from the center of symmetry they become negative -- a property usually
encountered for the dark energy component only. In order to mimic the observed
late-time accelerated expansion, the matter component must significantly depart
from standard dust, presumably ruling this subclass of Stephani models out as a
realistic cosmology. The only way to accept these models is to keep all
standard matter components of the universe including dark energy and take an
inhomogeneity parameter small enough.Comment: REVTEX4-1, 12 pages, 6 figures, explanatory material added, version
to appear in PRD, conclusions and results unchange
Cosmological tests of sudden future singularities
We discuss combined constraints, coming from the cosmic microwave background
shift parameter , baryon acoustic oscillations (BAO) distance
parameter , and from the latest type Ia supernovae data, imposed
on cosmological models which allow sudden future singularities of pressure. We
show that due to their weakness such sudden singularities may happen in the
very near future and that at present they can mimic standard dark energy
models.Comment: 8 pages, 2 references adde